Most chemical carcinogens induce DNA damage and are mutagenic at specific genetic loci; however, certain carcinogens (including the human carcinogens diethylstilbestrol (DES), asbestos, arsenicals and benzene) usually do not induce gene mutations. We have examined the ability of these chemicals to induce morphological transformation, gene mutations and chromosome mutations in Syrian hamster embryo (SHE) cells in culture. We have previously proposed that the mechanism of action of DES is related to its ability to induce numerical chromosome changes, i.e., aneuploidy. Currently, DES-induced aneuploidy is being examined in the newborn mouse genital tract to test whether these changes occur in vivo in the target tissue. The mechanism of another important human carcinogen, asbestos, was also examined. We have proposed that asbestos induces cell transformation due to its ability to induce chromosomal changes. We have identified a possibly novel transforming oncogene in human mesotheliomas, and currently we are cloning this gene. Sodium arsenite and sodium arsenate are inactive as gene mutagens but are potent inducers of cell transformation but is a weak gene mutagen. This chemical is a very effective inducer of aneuploidy in this system. These results further support our hypothesis that cell transformation involves a chromosomal mutation and suggest an important role for carcinogen-induced aneuploidy in carcinogenesis. Di(2-ethylhexyl)phthalate (DEHP), a commonly used plasticizer, induces peroxisome proliferation in liver cells and hepatocellular carcinomas in rodents. We have shown that DEHP induces morphological transformation, chromosome aberrations, and peroxisome proliferations of cultured Syrian hamster embryo (SHE) cells. The transformation frequency and chromosomal aberrations by DEHP was enhanced in the presence of rat liver post-mitochondrial supernatant. The results suggest a possible involvement of genetic damage by DEHP metabolites in the induction of transformation of SHE cells. No clear relationship between induction of peroxisome proliferation and cell transformation was observed.